Among existing approaches for distributed sensing, the fiber optic paradigm is particularly unique, given its numerous advantages, including immunity to electromagnetic interference, miniature sizes, harsh environment operation, and capability of large-scale multiplexing.
One application is an optical fiber distributed acoustic sensor (DAS) designed using phase-sensitive optical time domain reflectometry (ϕ-OTDR). The ϕ-OTDR based DAS has been deployed in applications such as real time structural health monitoring, oil and gas industry, and aerospace transportation. Its operation principle consists of launching a train of optical pulses generated by a narrow linewidth laser into a fiber under test (FUT). At the FUT input port, consecutive Rayleigh backscattered traces are recorded in the time domain. Each Rayleigh trace has a speckle-like profile because of coherent interference of the signals reflected by scattering centers within the injected pulse duration. In the absence of intrusion along the FUT, (such as no refractive index perturbation) the recorded Rayleigh traces are ideally identical. When an acoustic signal is applied at a position along the FUT, the effective refractive index changes at this position, and consequently the intrusion of vibrations from the acoustic signal can be sensed by observing the intensity fluctuation of a corresponding speckle in the recorded traces.
Another application is distributed temperature sensing (DTS). Its operation principle consists of launching a train of optical pulses into the FUT. The pulses are backscattered by thermal excitement of the FUT. At the FUT input port, backscattered Stokes Raman and anti-Stokes Raman signals are compared to determine the temperature of the FUT.
The backscattered power of the Raman signal within an optical fiber is roughly 40 dB (decibels) less than that of the Rayleigh signal. Consequently, in DTS systems, the pump powers that are employed can induce nonlinearity in single mode fibers (SMFs). This fiber nonlinearity, such as stimulated Raman scattering, inhibits the proper operation of DTS systems. Alternatively, multimode fibers (MMFs), which exhibit relatively greater nonlinearity threshold in comparison with SMFs, are used for DTS systems. However, it is difficult to offer MMF-based DAS system because MMF supports propagating huge numbers of modes, and each mode encounters independent interference behavior that is detrimental to Rayleigh signals.